The fact that junk food could provoke this response isn’t entirely surprising, says Gene-Jack Wang, MD, the chair of the medical department at the U.S. Department of Energy’s Brookhaven National Laboratory, in Upton, N.Y.

“We make our food very similar to cocaine now,” he says.

Coca leaves have been used since ancient times, he points out, but people learned to purify or alter cocaine to deliver it more efficiently to their brains (by injecting or smoking it, for instance). This made the drug more addictive.

According to Dr. Wang, food has evolved in a similar way. “We purify our food,” he says. “Our ancestors ate whole grains, but we’re eating white bread. American Indians ate corn; we eat corn syrup.”

The ingredients in purified modern food cause people to “eat unconsciously and unnecessarily,” and will also prompt an animal to “eat like a drug abuser [uses drugs],” says Dr. Wang.

The neurotransmitter dopamine appears to be responsible for the behavior of the overeating rats, according to the study. Dopamine is involved in the brain’s pleasure (or reward) centers, and it also plays a role in reinforcing behavior. “It tells the brain something has happened and you should learn from what just happened,” says Kenny.

Overeating caused the levels of a certain dopamine receptor in the brains of the obese rats to drop, the study found. In humans, low levels of the same receptors have been associated with drug addiction and obesity, and may be genetic, Kenny says.

However, that doesn’t mean that everyone born with lower dopamine receptor levels is destined to become an addict or to overeat. As Dr. Wang points out, environmental factors, and not just genes, are involved in both behaviors.

Dr. Wang also cautions that applying the results of animal studies to humans can be tricky. For instance, he says, in studies of weight-loss drugs, rats have lost as much as 30% of their weight, but humans on the same drug have lost less than 5% of their weight. “You can’t mimic completely human behavior, but [animal studies] can give you a clue about what can happen in humans,” Dr. Wang says.

Although he acknowledges that his research may not directly translate to humans, Kenny says the findings shed light on the brain mechanisms that drive overeating and could even lead to new treatments for obesity.

“If we could develop therapeutics for drug addiction, those same drugs may be good for obesity as well,” he says.